Is there a device similar to a metal detector that can find rocks that are above a certain hardness?

If you know the approximate area where she lost the stone, you can run the sand through a fine grain sieve and hopefully collect the gem in the sieve

Not that I know of. The only way I can see you finding it is to go back to the area last remember seeing the ring with the stone in it. It may have come loose some where else like in her purse or handbag, a pocket, the car. But if it's that important, you dig for it and sieve the sand. The metal detector is still a good idea though even if you don't find the stone you could find a whole ring with a sapphire on it. Hey it's your moms ring, what ever it takes ye.

Just reading your question title I was about to suggest a hammer. But then reading the details I think there is none.